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Gone with the wind
Over the August bank holiday weekend, Network Rail successfully replaced two bridges over the River Arun, work which formed part of a £6.2 million investment to upgrade the railway for passengers using the Arun Valley line.
Both bridges - one known as Peppering Bridge and the other Black Rabbit Bridge - are located between Arundel and Amberley stations, about 1.5km north of Arundel in low-lying and flood-prone farmland. The area borders two sites of special scientific interest (SSSI) and they are contained within a site of nature conservation importance (SNCI). Phil Avery, Network Rail's project manager for the scheme, explained that they have been working very closely with the Environment Agency and Natural England to plan the project.
The bridges are half-a-mile apart and identical twin structures. Each has three spans with intermediate concrete piers and brick abutments. The materials used for their construction are the same as is their geometrical layout. They comprise wrought iron outer main girders and a replacement steel l-section centre girder with under-slung cross girders carrying longitudinal timbers; these support the flat-bottomed track. There can't be too many bridges of this design left and, inevitably, assessment has determined that both structures were under strength for the route, requiring restrictions for certain types of traffic. It was decided that the most viable option was renewal of the superstructures whilst retaining the existing abutments and piers.
Different options considered
Atkins carried out the development work and outline design. Different options were considered - whether to go for one continuous span or three separate ones as well as the preferred type of deck. They also reviewed the condition of the abutments and piers. The latter were strengthened in the1930s and considered suit‑able but there was some uncertainty over the abutments.
It was known that they were founded on timber piles but very little detailed information was available. lt was finally decided to use three spans - the piers being deemed adequate, as were the abutments, on the proviso that a monitoring system was installed using tilt meters so that any subsequent movement could be detected and monitored.
BAM Nuttall is Network Rail's framework contractor for all civils work in this part of the country and it was invited to carry out the reconstruction work under the guidance of its project manager Gareth White and his team. The services of Cass Hayward were procured to do the detailed design work.
The two new superstructures have a life expectancy of 120 years. Both decks consist of three spans of U-type composite steel and concrete. Six spans are required per bridge, each one carrying a single track. The steel fabrication work was carried out by Allerton Engineering and the 12 decks were transported to site by road so that the concrete deck could be cast and the structures painted before installation. Each deck now weighed approximately 40 tonnes.
Given the location, it was imperative that the work was carried out during a period of dry weather. Don't laugh! So a plan was developed to replace both structures over two consecutive weekends during the August bank holiday. The first would be used to carry out advanced work during a 28- hour possession whilst the second, involving a 76-hour possession, would see the decks replaced.
Temporary buffer stops
As luck wouldn't have it, the work coincided with the second week of the Arundel festival. Following consultation with West Sussex County Council and Arundel Town Council, Network Rail and Southern Trains introduced special train arrangements. Temporary buffer stops were installed to enable trains to continue running via the coastal route into Arundel Station over the bank holiday weekend. Supporting rail replacement bus services were also put into operation from nearby Pullborough. Throughout the work, these arrangements worked well for all concerned.
It was decided that all plant and materials would be brought to site by road. The nearest road to the bridges was a country lane about half-a-mile away so they had to construct two haul roads across the low-lying fields and build suitable compound areas for both sites. Adequate working areas were required to prepare the new steel bridge decks as well as facilities for on- site casting of concrete decks and cill beams.
A decision was made to trial the erection of the bridges to ensure that all went well when the work was done for real. It was determined that a 1,200- tonne Sarens fixed jib road-mobile crane would be used for Black Rabbit Bridge whilst a similar 800-tonne crane would take the strain at the Peppering site since it could be positioned closer to the structure. As a consequence, substantial temporary works for both cranes had to be installed and ready in advance of the work. To round things off, there was the inevitable cluster of HV and S&T cables that needed to be moved onto temporary cable structures. These were constructed during the 28-hour possession on the 23rd and 24th August, with the cables being successfully slewed onto them.
Some pre-cutting work was carried out on the abutments and piers as well as laying out bags of ballast at the ends of the bridges. Everything went well and in accordance with the plan. lt was imperative that the weather held out for the big weekend and that the wind was gentle during the big lifts. Phil Avery and the team were feeling quietly confident but they were not taking anything for granted. Although success would rely on the next three working days, it had taken Phil and his team three years to prepare for this.
Difficulty cutting out pockets
So everything was ready for the 76- hour possession planned for the August bank holiday. It started with the installation of the buffer stops, enabling trains to run into Arundel Station. Plant and the cranes were put in position; the old track then being cut up and taken away along with the bridges' longitudinal timbers. Both bridges were removed, paving the way for one of the trickiest tasks - the reduction and preparation of the abutments and piers for the installation of the precast concrete cill beams together with insitu concrete blocks in the piers ready to accept the new bearings.
The main concern was the lack of detailed information about the concrete piers so specialist concrete- cutting subcontractor Kilnbridge was brought in to undertake the work. What would they find when they started to cut? Concerns proved to be justified when one of the 1.5m pockets - being cut out using a special wire cutter - took 18 hours to complete. Fortunately, the remaining pockets were cut out in five or six hours which was more in line with the programme expectations.
The rest of the work went according to plan; each bridge had its six replacement decks installed plus new cills, pier blocks and ballast walls, backfilled behind the abutments and waterproofed. Gamble Rail carried out the trackwork, installing the bottom ballast and track panels; both the running and DC rails were welded. Walkways were added, cables slewed back into new routes and top ballast run out before tamping took place. S&T and HV testing/commissioning were completed without fuss, allowing the temporary buffer stop to be removed and the line handed back within the time allocated.
The weather was fine - there were no strong winds. lt did become gusty for a few hours though not sufficiently to hold up the work. Unfortunately, due to a considerable change in day-to-night temperature, one rail joint had to be clamped so the opening TSR speed was confined to 20mph on the Down line rather than 40mph as planned.This was rectified the following night.
A job well done
Network Rail took full advantage of the possession by successfully completing steel strengthening work on Timberley Viaduct, located north of the two bridges. The whole project has been completed without mishap, delay or accident. According to PhilAvery, "it went exactly as we programmed"
It just goes to show that if you pay attention to detail, involve local people and agencies in your plans, understand the priorities of others - in this case, the Arundel festival organisers - work closely with your client, Southern Rail, and continuously review both your plans and the associated risks, you can rebuild a bridge in relatively little time. If you are really smart, you can rebuild two. It's a credit to Phil and all the team involved in this impressive piece of work.
Article courtesy of Rail Engineer - oct 2009
Further Info
Peter Bishop - Head of Public Relations & Corporate Communication
BAM Nuttall Limited
St James House, Knoll Road, Camberley,
Surrey GU15 3XW
Tel: 01276 63484
Fax:01276 66060
peter.bishop@bamnuttall.co.uk
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